1. Field of the Invention
The present invention relates to a master cylinder for supplying a brake fluid to a brake apparatus of an automobile or the like.
2. Description of the Related Art
As a conventional master cylinder, as described in a microfilm of A Japanese Utility Model Application No. 152602/1980 (Japanese Utility Model Laid-Open No. 73248/1982), there is provided a constitution in which by carrying out so-to-speak fast fill for supplying a large volume of a brake fluid at an initial stage of operation when the brake fluid is supplied to a brake apparatus such as a disk brake or a drum brake, an ineffective fluid amount at an initial stage of stroke is compensated for, and as a result, a pedal stroke can be shortened (fast fill type master cylinder).
The fast fill type master cylinder is provided with a stepped cylinder having a large diameter cylinder portion and a small diameter cylinder portion, a stepped piston having a large diameter piston portion slidably inserted into the large diameter cylinder portion of the stepped cylinder and a small diameter piston portion slidably inserted into the small diameter cylinder portion of the stepped cylinder, and a cup seal (reverse flow checking opening and closing portion) for partitioning the inside of the stepped cylinder into a large diameter pressurizing chamber on the large diameter piston portion side and a small diameter hydraulic chamber on the small diameter piston portion side and permitting the brake fluid flow only from the large diameter pressurizing chamber side to the small diameter hydraulic chamber side.
Further, when the stepped piston is slidingly moved toward the small diameter hydraulic chamber by an input from a brake pedal, a volume of the large diameter pressurizing chamber is reduced by the sliding movement of the stepped piston, so that the reverse flow checking opening and closing portion is opened to supply the fluid from the large diameter pressurizing chamber side to the small diameter hydraulic chamber side the above-described fast fill, which carries out.
Further, the fast fill type master cylinder is provided with a relief valve for escaping the brake fluid from the large diameter pressurizing chamber to a reservoir when an inner pressure of the large diameter pressurizing chamber becomes equal to or higher than a predetermined value, and a notch is provided at the relief valve to always communicate the large diameter pressurizing chamber to the reservoir. Through the communication path of an extremely small diameter due to the notch, the fluid is supplied form the reservoir to the large diameter pressurizing chamber.
Meanwhile, there poses a problem that since the above-described master cylinder is provided with the notch, when the pressure is elevated slowly, the fluid of the large diameter pressurizing chamber flows to the reservoir and the fast fill cannot be carried out sufficiently.
Further, according to the above-described master cylinder, when the inner pressure of the large diameter pressurizing chamber becomes equal to or higher than the predetermined value, the relief valve is opened and the brake fluid is quickly escaped from the large diameter pressuring chamber to the reservoir. Therefore, when the pressure is elevated at high rate, namely when in operation of the brake pedal is stepped at comparatively fast, the hydraulic pressure of the large diameter pressurizing chamber is quickly released by the opening operation of the relief valve when hydraulic pressure of the large diameter pressurizing chamber is elevated to the predetermined pressure and therefore, a pedal stroke is extended without being accompanied by pedal reaction force and the piston is moved to the small diameter hydraulic chamber side. This has brought about a strange feeling in the pedal operation that the vehicle speed is reduced without a real feeling that the pedal is depressed.
Further, there is another conventional fast fill type master cylinder having a cut-off portion on the large diameter pressurizing chamber side for cutting communication between the large diameter pressurizing chamber and the reservoir by being closed by the sliding movement to the small diameter hydraulic chamber side of the stepped piston and a cut-off portion on the small diameter hydraulic chamber side for cutting communication between the large diameter pressurizing chamber and the small diameter hydraulic chamber by being closed by the sliding movement to the small diameter hydraulic chamber side of the stepped piston in order to promote the fast fill function. In order to promote the fast fill function in such a master cylinder, it is necessary to make an ineffective stroke of the stepped piston until the cut-off portion on the small diameter hydraulic chamber side and the cut-off portion on the large diameter pressurizing chamber side are brought into a closed state, as short as possible.
However, when the ineffective stroke is simply set to be short, sectional areas of flow paths of the cut-off portion on the side of the small diameter hydraulic chamber and the cut-off portion on the side of the large diameter pressurizing chamber, are narrowed. When the sectional areas of the flow paths are narrowed in this way, in the case that the master cylinder is used in a combination with a traction control apparatus, there poses a problem that when the brake fluid is forcibly sucked from the reservoir via the master cylinder in order to make the traction control apparatus operate the brake apparatus, the brake fluid cannot be made to flow at a sufficient flow rate, that is, in high flow.
It is noted that the traction control apparatus is an apparatus that, when the brake pedal is not operated, a driver excessively operates an accelerator on a slippery road and causes a wheel spin at a drive wheel of a vehicle, controls the drive wheel by forcibly supplying the brake fluid from the reservoir to a brake apparatus such as a wheel cylinder via the master cylinder, thereby to reduce the wheel spin. The master cylinder used in the combination with the traction control apparatus must have a function capable of making the brake fluid flow to the traction control apparatus in high flow (high flow function) in an initial state.
Therefore, the applicant has conceived that a structure of previously filed Japanese Patent Application No. 294502/1998 (equivalent to U.S. Pat. No. 6,272,858 B1) to the cut-off portion on the small diameter hydraulic chamber side and the cut-off portion on the large diameter pressurizing chamber side. The structure is to close a port opened at an outer peripheral portion of the piston by a cup seal by the sliding movement of the piston. By forming a control taper face, a front side of which is smaller in diameter, rearward from the opening portion of the port at the outer peripheral portion of the piston, even when the entire of the cup seal does not completely pass over the port in the sliding movement of the piston, a rear end portion of the cup seal rides over the control taper face to thereby increase face pressure and close the port. The structure can shorten the ineffective stroke of the piston while achieving the high flow.
However, when the above-described structure is applied to the cut-off portion on the small diameter hydraulic chamber side and the cut-off portion on the large diameter pressurizing chamber side, the control taper face are required in both of the cut-off portions, further, it is necessary to strictly control positional accuracy of both of the ports and therefore, there poses a problem that cost is increased.